1. This is a French rule made round about 1875.
2. What makes it special is the fact that, rather than having single cycle logarithmic scales, it has half-cycle scales; this gives 50cm accuracy on a 25 cm body. This can be seen on the detailed views where the upper scales go from 1 to 3.16 (Ö10) and the scale immediately below it goes from 3.16 to 10.
3. The rule also has two slides. With a total of 4 scales on the stock and 4 scales on the slides this gives every possible combination of scale and displacement (1 to 3.16 stock v 1 to 3.16 slide, 3.16 to 10 stock v 3.16 to 10 slide, 1 to 3.16 stock v 3.16 to 10 slide and 3.16 to 10 stock v 1 to 3.1 slide).
4. On the back there is a scale of sines and tangents in three ranges ( approximately 0° to 20°, 20° to 42° and 42° to 90° for sine and 0° to 19°, 19° to 33° and 33° to 45° for tan). Between the angles are the numbers 1 to 100 and at the top left hand corner on the back "r=100" to remind users that the indicated values have to be divided by the ratio (?) of 100.
5. One of the back slides has logarithmic scales (upper slide) which are double length to work with the double length scales on the front (this can be seen from the 100(0) and 500 in the detail below). The other has a standard single cycle C scale.
6. Some Tavernier Gravet rules have double chisel type cursors (see figure below). This enables calculations to be carried out close to either end of the rule. Why this rule, which has scales which go right to both ends of the rule, does not have such a cursor must remain a mystery. The only solution I can offer is that with 4 pairs of scales one of them would have been well placed relative to the cursor.
My thanks to Jim Bready for providing the images of the rule and additional information.
Front view
Detail - front left
(not from this rule)
